This invention relates generally to thermal treatment of various compositions, materials and/or material systems and, more particularly, to such treatment without external stresses and without phase transformation.
Thermal treatments, both with or without imposed external stresses, are commonly used to densify materials which contain internal pores or cavities. For instance, despite their outstanding creep resistance, oxide-dispersion-strengthened aluminum materials have limited creep ductility due to the formation and subsequent growth and linkage of creep cavities in service.
One approach to extending the creep-life of various engineering components is ex-situ treatments to close creep cavities after some service time. The technical literature contains many examples of the shrinkage and closure of cavities formed during service by isothermal heat treatment with or without superimposed hydrostatic pressure. Although isothermal heat treatment at ambient pressure is simple and inexpensive, the time required to fully close creep cavities is often prohibitive. Hot isostatic pressing (HIP) can more rapidly close porosity, but at an increased cost. Further, studies show that cavity shrinkage is strongly affected by the internal residual stress state, potentially increasing the rate of densification or causing cavity growth rather than shrinkage.
The technical literature provides another approach: densification of polymorphic metal powders by a process of thermal cycling while simultaneously applying an external stress. The related deformation mechanism for densification is referred to as transformation superplasticity, and results in a solid/solid phase transformation upon application of the external stress.